Relaxin is a protein hormone secreted primarily by the ovaries of mammals during pregnancy, its richest source being the ovaries of pregnant sows. Its role in enhancing birth of the fetus by enlarging the birth canal is accepted, although the intricate role it plays in synergism with other hormones is not well understood. Because of its increased secretion in parallel with increased mammary gland development during pregnancy, the possibility of its stimulating effect on mammary gland growth should be investigated. To assess mammary growth, the chemical determinations of DNA and RNA, as well as the classical histological and whole-mount observations, will be employed on mammary glands in rats. Relaxin will be injected into ovariectomized or hypophysectomized-ovariectomized mature virgin rats each day for 19 days. On the twentieth day, thoracic mammary glands will be removed for histological and whole-mount observations, while abdominal-inguinal glands will be weighed and nucleic acids determined. Groups of 15 rats will be used to assess the role of relaxin alone and in synergism ovarian and pituitary hormones in stimulating mammary gland development. Ovariectomized groups will include: virgin controls, relaxin at 3 different levels, progesterone alone and with relaxin, estradiol alone and with relaxin, and combinations of all 3 hormones. Hypophysectomized-ovariectomized groups will include: the above groups plus those receiving prolactin in combination with either or all of the ovarain hormones (including relaxin), and those receiving somatotrophin in combination with the ovarian hormones. The results will provide a qualitative (histological) and quantitative (nucleic acids) index of mammary growth as affected by relaxin in synergism with other hormones.